The prevalence of dangerous strains of the human papillomavirus - the most common sexually transmitted infection in the United States and a principal cause of cervical cancer - has dropped by half among teenage girls in the last decade, a striking measure of success for a vaccine that was introduced only in 2006, federal health officials said Wednesday.

Infection with the viral strains that cause cancer dropped to 3.6 percent among girls ages 14 to 19 in 2010, from 7.2 percent in 2006, a new study has found. The vaccine protects against strains of the HPV virus that cause 70 percent of cervical cancers.

The study, published in the June issue of the Journal of Infectious Diseases, covered the years 2003 to 2010. Its findings were based on a national survey that is conducted every two years and considered the gold standard on health indicators.

The decline surprised public health experts because vaccination rates in the U.S. are still relatively low. Only about a third of teenage girls have been vaccinated with the full course of three doses. Health officials offered several possible explanations for why the drop was so sharp despite the fact that most teenage girls in the U.S. are still not fully vaccinated.

One possible reason is a phenomenon known as herd immunity, in which vaccinated people reduce the overall prevalence of the virus in society, reducing the chances that unvaccinated people would have sex with someone who is infected. Another is the unexpected effectiveness of a partial dosage of the vaccine.

About 79 million Americans, most in their late teens and early 20s, are infected with HPV. Each year, about 14 million people become infected. The virus causes about 19,000 cancers in women every year, and 8,000 in men, according to the CDC.